This invention relates to a self-cooling container for water or other beverage. The wall of the container comprises a non-porous permeable membrane permitting transmission of water vapor. Evaporative cooling keeps the temperature of the water in the container below ambient.
Various approaches have been proposed to cool portable water containers in the absence of a conventional refrigeration device. A simple and effective system would be of great value for general outdoor use but unfortunately none of the methods suggested in the prior art meets the necessary requirements of simplicity and functionality.
A method proposed involves the use of an endothermic reaction to cool a liquid that is in contact with the water container. The method is undesirable because it is impractical, costly, and it involves the use of chemicals that may present a safety hazard. Furthermore, it provides a cooling effect that is only temporary.
The approach claimed in U.S. Pat. No. 5,983,662 utilizes evaporative cooling. A standard can is placed into a perforated structure having a layer of sponge disposed along its inner suface. The sponge material is kept wet with water. As the water evaporates from the sponge, the evaporative process draws heat from the surface of the can thus cooling its content. This method has several disadvantages. The construction of the device is relatively complex. Water must be frequently added to the sponge to maintain the evaporative process. Cooling is inefficient because heat is drawn from ambient air as well as from the can.
Other approaches utilize cooling by evaporation of the water present in the container itself. It has been known for a long time that canvas bags made of certain fabric materials such as flax cloth have the peculiar property of providing controlled seepage of water through the cloth to the outer surface. Evaporation of the water from the outer surface of the bag into the environment provides a cooling effect. U.S. Pat. No. 2,467,792 claims a similar effect from bags made of non-flax cloth, preferably cotton, impregnated with a material capable to fill the interstitial spaces of the fabric to limit exudation of water to the extent required to maintain a thin film of water on the outer surface without excessive oozing and dripping. U.S. Pat. No. 4,368,766 claims water containers with a porous wall having maximum pore size of 0.1-50 microns, preferably less than 5 microns. Continuously porous, expanded polytetrafluoroethylene film is said to be the preferred material, optionally laminated to a fabric on one or both sides. The fine pore size allows passage of water vapor but not liquid water, thus a cooling effect is produced while the outer surface of the container remains dry.
The problem with all containers utilizing porous or microporous walls to provide evaporative cooling is the potential for contamination. These containers permit evaporation because of tiny holes present in the container wall. A portable water container used recreationally on hiking and camping trips or by the military in field operations is likely to be exposed to dirt, rain and mud. Bacteria could find their way into the container if the outside surface of the container is exposed to dirty water. The problem could be aggravated if the container is exposed to dirty water while temporarily empty so that the driving force for water passage is from the outer to the inner surface.
Thus, a need exists for a portable water container capable of maintaining water below ambient temperature by evaporative cooling while assuring that the purity of the water is maintained with no possible effect of outside contaminants.